Electroheating, also called "ohmic heating ", "resistive heating" and "heating by electroconductivity" is a process wherein an electrically conductive material is heated by passing an electrical current through the material, so that electrical energy is converted to heat by the resistance of the material. Stated another way, the material itself acts as a resistance heater. Typically, the electrical current is passed through the material by applying an electrical potential to spaced apart electrodes in contact with the material.
Because heat is evolved within the material itself, the rate of heating is not limited by the rate of heat transfer through the boundary of the material, or by the rate of conduction within the material itself. Thus, electroheating theoretically can provide rapid and uniform heating.
One important application for electroheating is in pasteurization and similar bacteriological controlled processes applied to fluid foods and biological materials such as milk, whole egg, egg and milk products, soups, stews and the like. Industry practice and governmental regulations for processing such products typically require that the product be brought to a required minimum temperature and held at or above the required minimum temperature for a specified minimum holding period. However, many such products are thermally sensitive and if held near or above the required minimum treatment temperature for prolonged periods, the flavor, texture or usefulness of the product may be adversely affected. Accordingly, the product typically is preheated to a temperature below the required minimum temperature either by a conventional heat exchanger such as a plate heat exchanger, scraped surface heat exchange or the like. After preheating, the product passes through an electroheating apparatus where it is rapidly heated to above the required minimum temperature. After passage through the electroheater, the product passes through a holding zone, typically a long, thermally insulated pipeline. If the product is of a proper temperature at the exit of the holding zone, it is cooled rapidly and then packaged.
There are, however, substantial practical difficulties with electroheating of thermally sensitive materials. Passage of an electric current through many materials tends to cause electrolysis as, for example, conversion of water and salts in a food product to the constituent gases. This, in turn, tends to promote undesired side reactions in the food product leading to off-flavors. Further, with some electrode materials, electrolytic effects can lead to gradual dissolution of the electrode and contamination of the product with material from the electrode. As disclosed in U.S. Pat. No. 4,729,140, electrolytic effects in electroheating can be suppressed effectively by applying the electrical current at a frequency above the 50 cycle or 60 cycle AC commonly available from the power mains as, for example, a frequency between about 100 Hz and about 450 KHz. Typically, frequencies on the order of 100 KHz or more, in the radio frequency or "RF" range, are used. Although the use of RF power in electroheating does effectively suppress electrolysis, it requires costly frequency conversion apparatus.
Moreover, to avoid problems of product damage such as coagulation in the case of eggs and formation of local arcs within the apparatus, the energy input to electroheating apparatus has been limited. Merely by way of example, PCT Publication WO93/19620 suggests specifically that electroheaters should be operated so that the product temperature is raised within each electroheater by no more than about 15.degree. C. and most preferably by more than about 5.degree. C. and further teaches, as an example, a heater which can raise the product temperature only at the rate of "1.degree. C. per second", a rate no higher than that achievable with conventional plate heat exchangers. Thus, heretofore, one approach which has been taken to achieving satisfactory results in electroheating has been to operate at relatively low current densities and, typically, at relatively low rates of energy input. Further electroheating apparatus and methods are described in U.S. Pat. No. 5,290,583.
Other electroheating apparatus which may be useful incorporate a series of concentric electroheating cells each incorporating a generally cylindrical outer electrode and rod-like inner electrode concentric with the outer electrode. In each such cell, the electrical potential is applied between the central, rod-like electrode and the outer electrode, so that the potential difference is generally radial. The apparatus may further include elongated "sight glass" cells, each including a dielectric pipe and electrodes contacting the fluid at opposite ends of the pipe. In one particularly useful arrangement the fluid to be heated first enters through one sight glass cell, then passes through the concentric cells in sequence and leaves the apparatus through the other sight glass cell. These sight glass cells operate at a relatively high potential, such as about 7800 volts whereas the concentric cells operate at about 200 volts potential. The concentric cells are connected electrically in parallel with one another. Apparatus of this nature using radio frequency ("RF") has been used successfully in treating whole egg and egg products. Millions of pounds of product have been successfully pasteurized using this electroheating apparatus and the resulting product has been widely accepted as having excellent flavor characteristics and storage stability.
Accordingly, despite the significant efforts and progress in the electroheating art there have still been significant needs for further improvements.